An absorbent article such as a pants-type disposable diaper and the like includes a front waistline portion for the wearer's front waistline, a back waistline portion for the wearer's back waistline and a crotch portion for the wearer's crotch. On each side of the crotch portions, a leg-surrounding opening portion (leg hole, for example) through which a wearer's leg is inserted is formed.
As a method for manufacturing such absorbent article, the method described below is disclosed. Specifically, stretched string-like elastic members are joined to a pair of webs (hereinafter simply referred to as “webs”) in the conveyance direction (that is, in the machine direction), the webs including a continuum of the front waistline portions and a continuum of the back waistline portions. Then, a member for the crotch portion (crotch portion member) is joined to the pair of webs so that a longitudinal direction of the crotch portion member is approximately in parallel with the cross direction (CD) orthogonal to the machine direction.
Next, the web is folded into two approximately at the center of the crotch member so that a side edge of one web can be brought to overlay a side edge of the other web. Thereafter, the web is cut in the cross direction at predetermined intervals (to the size of a product) in the machine direction to form a pants-type absorbent article (refer to Patent Document 1, for example).
A machine configured to fold a pair of webs including joined crotch members into two (hereinafter referred to as “folding machine”) includes at least a large-diameter roll, a folding base point bar and multiple guide rolls.
The large-diameter roll presses one of the webs. The folding position bar presses a base point for folding the webs at the folding position (approximately at the center of the crotch member) by bringing the other web toward the one web. The multiple guide rolls guide the other web toward the one web pressed by the large-diameter roll. In this configuration, the webs being supplied successively are folded into two to form pants-type absorbent articles.
In such folding machine, a sensor or the like detects positions of a side edge of one web and a side edge of the other web being conveyed, and thereby manufacture defect of the absorbent article is prevented from occurring due to a misalignment between a side edge of the one web and a side edge of the other web in the cross direction.
In a folding machine disclosed by the Patent Document 1, a side edge of one folded web is in alignment with a side edge of the other web without misalignment in the cross direction. However, there is a difference between stresses being applied to the webs.
In particular, since the one web comes into contact with the large-diameter roll and the other web come into contact with of the multiple guide rolls, stresses to be applied to the webs may differ from each other. Also, an absorbent article is asymmetric with respect to the folding position, due to factors such as differences in external appearance and stresses by the elastic members between the front waistline portion and the back waistline portion, and the arrangement position of the crotch portion member. Thus, different stresses are likely to be applied to the two webs, respectively.
As a result, the side edges of respective two webs are conveyed while following different tracks with different lengths, or are conveyed under different levels of tensions. Due to such differences, a minor phase misalignment may occur between the two webs. Here, the phase misalignment refers to a state in which the side edge of one web is aligned with the side edge of the other web after both webs are folded, but a center line in a longitudinal direction of the crotch portion member is misaligned.
According to the folding machine disclosed by the Patent Document 1, revolution speed or the like of both of the large-diameter roll and the multiple guide rolls are adjusted so that the phase misalignment falls within an allowable range according to the conveyance speed and characteristics of the webs. For this reason, when the web conveyance speed is changed or a web is replaced, for example, the phase misalignment described above might become so conspicuous as to be regarded as a product defect. Also, it is cumbersome to readjust the revolution speed or the like of the large-diameter roll and the multiple guide rolls whenever needed.